In order to protect a wiring pattern formed on a substrate by a method such as screen printing from an external environment or in order to protect a printed circuit board from the adhesion of solder on its part where soldering is not necessary in a soldering process performed when electronic components are surface-mounted on the printed circuit board, a protective film called a cover coat or a solder mask is used. This protective film covers the printed circuit board and exhibits a protecting function.
As a material of such a protective film, for example, an epoxy resin composition has been used for a thermosetting solder resist, but it has been difficult to use the epoxy resin as it is because of its easy flammability. Therefore, in order to give flame retardancy to the resin composition, a halogen-based flame retardant is added or brominated epoxy resin which contains bromine in its skeleton is used (see, for example, JP-A 9-054434 (KOKAI)).
However, after it was reported that decabromodiphenyl oxide which is the most typical bromine-based retardant generates toxic brominated dibenzoxide and furan when it is burned, there has arisen a doubt about safety of all the bromide-based flame retardants. Further, when resin using the conventional bromine-based flame retardant is burned, toxic gas such as dioxins is produced, which gives a great load to the environment when it is burned and when it is discarded.
Further, in order to cope with an environmental problem, an organophosphorus-based flame retardant has recently been used instead of the bromine-based flame retardant. However, the organophosphorus-based flame retardant is not very excellent in flame retardancy, and has to be added in more amount than the bromine-based flame retardant in order to give the same level of flame retardancy to a resin composition, and thus involves problems such as property deterioration of the composition and bleeding.
Reference 1: JP-A 9-054434 (KOKAI)